For electrically connecting conductors of a plurality of wires to each other, for example, a method is adopted in which insulating coatings are removed to expose conductors and the exposed conductors are soldered to each other or the conductors are twisted and plied together by manual labor or the like. Particularly in recent years, a method in which the conductors are welded to each other by ultrasonic welding has been widely adopted, because an operation can be easily and surely performed (for example, Patent Document 1).
Meanwhile, Patent Document 2 points out that performing the ultrasonic welding by the method disclosed in the Patent Document 1 causes a problem in connection strength which may separate conductors from each other. As a connection method for solving this problem, the Patent Document 2 discloses a connection method in which exposed conductors are twisted together in the same direction and this twisted portion is welded by ultrasonic welding.    Patent Document 1: Japanese Patent Application Laid-Open No. 2000-188018    Patent Document 2: Japanese Patent Application Laid-Open No. 2005-322544